Method and device for producing a hollow metallic billet from a metallic billet

ABSTRACT

The present invention is directed to a method for producing a hollow metallic billet ( 10 ) from a heated metallic billet ( 1 ) by a piercing operation (for example with the use of a piercing mandrel ( 4 )). The method is characterized in that a local temperature change is effected at least at the hole initiation end ( 11 ) of the billet ( 1 ) in at least one zone ( 110, 112 ) and the zone is rotationally symmetrical to the central axis (M) of the billet ( 1 ). Also described is a device for producing a hollow metallic billet ( 10 ) from a metallic billet ( 1 ) and comprising a holder ( 3 ) for the billet ( 1 ), characterized in that the device includes at least one temperature adjusting device or a projection element ( 2 ) operative to change the temperature of the billet ( 1 ) in the holder ( 3 ) at least zonewise and directed at a subzone of at least one of the end sides of the billet ( 1 ).

REFERENCE TO PENDING PRIOR PATENT APPLICATION

This patent application claims benefit of German Patent Application No. 10 2012 107 041.5, filed Aug. 1, 2012, which patent application is hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed to a method and a device for producing a hollow metallic billet from a metallic billet by a piercing operation, for example using a piercing mandrel.

BACKGROUND OF THE INVENTION

For the production of tubes, particularly seamless tubes, it is known to make a hole in a solid billet serving as the feedstock, thus creating a hollow billet that can be used directly as a tube or can be put through additional processing steps. Particularly in the production of steel tubes, hollow billets are made from heated solid steel billets in a single-stage or multi-stage continuous rolling process, and particularly in a forming unit for making holes in billets, for example a cross-roll piercing mill or a punch press, usually with the aid of a piercing mandrel.

The feedstock in such cases is usually a so-called round billet. Before being pierced, the billets are heated in a furnace to lower the yield stress and thus the stability in the entire billet to a sufficient extent so that the billet material can be shaped as well as possible during the subsequent piercing operation.

An essential and very significant factor, especially if the hollow billet is going to be used for metallic tubing, is a wall thickness distribution that is as uniform as possible. In the theoretical ideal case, the wall thickness is identical throughout the hollow billet. This case corresponds to an absolute eccentricity E_(absolute)=0 mm and thus also a relative eccentricity E_(relative)=0%.

The eccentricity of tubular objects is defined as the distance between the centers of the outer and inner circles of the tubular object. The absolute eccentricity (in mm) and the relative eccentricity (in %) can be calculated by means of the following formulas (1) and (2), respectively, the wall thickness of the tubular object being denoted by “s” and the subscripts “max” and “min” respectively denoting the maximum and minimum wall thickness of the particular cross section:

$\begin{matrix} {E_{absolute} = \frac{\left( {s_{\max} - s_{\min}} \right)}{2}} & (1) \\ {E_{relative} = {\frac{\left( {s_{\max} - s_{\min}} \right)}{\left( {s_{\max \;} + s_{\min}} \right)}*100\%}} & (2) \end{matrix}$

There is a need to keep the eccentricity of a hollow billet made from a billet to a minimum.

The heretofore-known technical measures for producing hollow billets or tubes with low eccentricity are based on changing the arrangement of or the distances between the tools. Examples of such measures are described in DE 3128055 C2, DE 3326946 C1, DE 4433397 C1 and EP 2067542 A1. Alternatively, it is proposed in the prior art to use additional or different tools. Such methods are described, for example, in DE 473723, U.S. Pat. No. 4,803,861, DE 19903974 A1, DE 3326946 C1, DE 4333284 C2 and DE 4433397 C1. It has also been proposed to mechanically produce recesses or pre-drilled holes on the central axis of the billet. This is described, for example, in DE 3328269 A1, GB 1008709, DE 2635342 C2, U.S. Pat. No. 4,052,874, GB 897015, DE 1247118 and GB 961796.

These measures are for the most part elaborate, cost-intensive and, in many cases, not sufficiently effective. Despite these technical measures, the eccentricity values obtained are not infrequently above 10%. A measure that is also commonly used is, after the completion of heat forming, to then cut away regions of increased eccentricity within the tube, for example particularly the ends of the tube. To determine the length of the tube end that shows increased eccentricity, it is first necessary to measure the eccentricity pattern along the tube, which entails greater metrological complexity. Excessive eccentricity values in the tube can, in the extreme case, lead to rejection of a large portion of the tube concerned, or even of the entire tube.

The object underlying the invention is, therefore, to create a solution by means of which eccentricity can be kept to a very low minimum reliably and in a simple manner during the production of hollow billets.

SUMMARY OF THE INVENTION

The invention is based on the realization that this object can be achieved by adjusting the stability of the billet so that this property defines and, where appropriate, guides the progress of the piercing and especially the orientation of a piercing tool in the billet during the piercing operation.

According to a first aspect, the object is therefore achieved by means of a method for producing a hollow metallic billet from a heated metallic billet by a piercing operation. The method is characterized in that a local temperature change is effected at least at the hole initiation end of the billet in at least one temperature change zone and the temperature change zone is rotationally symmetrical to the central axis of the billet.

According to the invention, the metallic billet used is preferably a metal billet of low-alloy or unalloyed steel. However, other metals, such as aluminum, for example, can also be used. The metal billet or metallic billet will also be referred to hereinafter as a billet. A hollow metallic billet, which will also be referred to hereinbelow as a hollow billet, is understood according to the invention to be an object produced from a metal billet and serving, for example, as feedstock in a subsequent method step for the production of metallic tubes, particularly steel tubes in a single-stage or multi-stage continuous rolling process employing one or more forming units. To make a hole in the billet and thus to create a hollow billet, according to the invention a forming unit is used, for example a cross-roll piercing mill or a punch press. The forming unit comprises, for example, at least one piercing mandrel. Whenever a piercing mandrel is referred to hereinbelow, the descriptions provided also apply to another tool of a forming unit serving to make holes. The creation of a hole in the billet to create the hollow billet will also be referred to hereinafter as a piercing method or piercing operation.

In the method according to the invention, the feedstock, i.e. the metal billet, can be a so-called round billet, that is, a solid cylinder with a circular-cylindrical or approximately circular-cylindrical cross section. However, the billet can also have another billet cross-section contour, for example a round or angular, polygon-like such contour. The billet can be supplied as a preformed or non-preformed round continuous casting of approximately constant diameter. The diameter of the round billet can be, for example, D=200 mm, and the weight of the billet can be, for example, m=100 to 1000 kg. The billet is preferably heated in a furnace, for example a rotary hearth furnace or a walking beam furnace, to a suitable temperature for subsequent forming. In the case of low-alloy or unalloyed steels, the billet is heated to a temperature of, for example, T=1300° C. or T=1200° C.

In the method according to the invention, a local temperature change is effected in at least one temperature change zone at the hole initiation end of the metal billet. “Hole initiation end” here signifies the end of the billet, and subsequently also of the hollow billet, at which the piercing operation begins.

The “zone” in which a temperature change is effected is understood to mean the region of the billet in which the temperature change is effected. This region or zone is thus referred to as a temperature change zone. The local temperature change preferably occurs in a temperature change zone of the hole initiation end that encompasses the end side of the billet. The temperature change zone preferably extends beyond the end face of the billet for a certain length in the axial longitudinal direction of the billet.

As previously described, before being pierced, the billet is, for example, heated in a furnace and is therefore used for the method as a heated billet. By the heating of the billet, the yield stress and thus the stability in the entire billet are reduced sufficiently so that the billet material can be formed as well as possible during the subsequent piercing operation. The objective is to create the most uniform possible temperature distribution throughout the billet. According to the invention, a local temperature change is effected.

“Local temperature change” here means a temperature change that occurs in only a portion of the billet. The temperature change zone in which the temperature change occurs thus has a different temperature from the other portions of the billet, particularly from the portions of the billet adjacent to the temperature change zone. Particularly preferably, the local temperature change occurs at least at one surface of the billet. Particularly preferably, the local temperature change occurs at one or both end faces of the billet. In addition, however, the local temperature change can also extend beyond the end face in the axial direction for a portion of the length of the billet.

The temperature change zone in which the local temperature change is effected preferably contains only a portion of the surface of the billet and further preferably extends for only a portion of the length of the billet. In particular, the temperature change zone in which the temperature change is effected contains only a portion of one of the end faces or of both of the end faces of the billet and a portion of the length of the billet, which portion is adjacent the particular end face. Under these conditions, the temperature change zone in the portion of the length of the billet adjacent the end face extends over only a portion of the cross section of the billet. In the case of a temperature change zone located in the end face of the billet on the periphery of said end face, the temperature change zone extends, for example, for a portion of the length of the circumferential surface of the billet.

By means of the temperature change zone that is created according to the invention, discrete temperature zones—that is, regions whose temperatures differ from one another—are formed on the surface and also in the interior of the billet. One of the temperature zones formed is the temperature change zone, whereas the other temperature zone or zones are defined by an initial temperature of the billet before the temperature change. The temperature in these additional temperature zones can then be, for example, the temperature exhibited by the billet after the initial heating.

The local temperature change is preferably effected by action on a portion of the surface of the billet. The action, which will be described in more detail below, is preferably limited to a portion of the end face and, where appropriate, additionally to a portion of the length of the adjacent circumferential surface of the billet. The action is preferably an active action on the surface of the billet. The effectuation of the local temperature change therefore differs from passive temperature changes, occurring for example as a result of cooling in the environment.

This means that only a portion of a surface of the billet is acted on, preferably a portion of the end face of the billet. The temperature change according to the invention therefore differs from heat treatments of the billet, such as heating or quenching, in which the entire surface is subjected to a temperature change.

Due to various influencing variables, the temperature distribution obtained in the billet, especially by conventional methods, is not completely uniform, i.e., the temperatures within a billet differ by, for example, 10 to 60 kelvin. These temperature differences are the adverse consequence of heating which by nature is never ideally uniform. As a result, the temperature distribution is fundamentally more or less non-uniform and the stability distribution in the billet is therefore also fundamentally non-uniform. With the use of the heretofore known technical solutions, this non-uniformity of temperature distribution is more or less pronounced, in terms of the magnitude and nature of the temperature distribution. Moreover, this non-uniformity of temperature distribution cannot be adjusted with sufficient precision.

Even a very slight non-uniformity of the stability distribution in the billet can cause very marked deficiencies in the quality of the tubular product. In particular, differences or fluctuations in stability in the end face of the billet, owing to the stochastically non-uniform temperature distribution, can have a strong impact on the piercing operation, especially during the first phase of the piercing process, the so-called hole initiation phase.

Non-uniformities of stability in the billet, together with non-uniformities of billet geometry, for example deviations from the ideal circular geometry of the billet cross section, can then cause quality defects, particularly dimensional deviations—increased eccentricity, for example—in the rolled product, hollow billet or finished tube.

Eccentricity is influenced by a number of physical variables, such as, for example, the stability and thus the shape-holding ability of a tool used for piercing, particularly the piercing mandrel, and its length. Eccentricity is also influenced by the properties of the billet, particularly the distribution of those properties in the billet. The temperature distribution and the stability distribution in the billet are especially important in this regard.

By means of the present invention, the local temperature change effected in a zone of the billet that is rotationally symmetrical to the central axis of the billet creates a graded and rotationally symmetrical temperature distribution in at least the end face of the hole initiation end of the billet. This is accompanied by a graded and rotationally symmetrical stability distribution in at least the end face of the hole initiation end of the billet. Compared to a conventionally produced hollow billet, the hollow billet produced in this way thus has a very uniform wall thickness distribution and thus very low eccentricity.

The fact that in the method according to the invention, the temperature change is effected at least at the hole initiation end and preferably in the so-called hole initiation zone of the hollow billet makes it possible to decrease the eccentricity of the hollow billet produced in the piercing operation, specifically primarily during the first phase of the piercing process, also known as the hole initiation phase. “Hole initiation zone” signifies according to the invention the end face at the hole initiation end of the billet, as well as a portion of the length of the billet which is adjacent to this end side and which can also be referred to as the hole initiation face. The portion of the length of the billet that is considered to belong to the hole initiation zone has a length that is smaller than the length of the billet. For example, the length of the portion of the billet that belongs to the hole initiation zone is preferably equal to no more than ⅓ or no more than ¼ of the overall length of the billet.

Reducing the eccentricity of a hollow billet fabricated in this way also results in an advantageous eccentricity distribution in the additional forming stages made from the billet, up to and including the finished tube. The essential technical disadvantage of high eccentricity of the rolled product or finished tube is that the attendant low wall thicknesses on one side in the eccentric cross sections concerned lead to a non-uniform, asymmetrical strain distribution on one side, and, in the extreme case—that is, where the wall thickness is too small—to overstraining of the particular cross section. Excessive eccentricity means either substantially increased rework or actual rejection of the tubular product.

The present invention makes it possible to keep eccentricity to a minimum in the production of hollow billets and, moreover, of precision tubing≧in the theoretical ideal case, to identically zero (E_(relative)=0%). In conventional production practice, eccentricity requirements are limited to a feasible value and are assigned to corresponding quality grades, with the result that eccentricities of, for example, E_(relative)<10% are permitted. A typical order of magnitude for relative eccentricity in the hole initiation zone of the billet is about E_(relative)=10% or higher. The eccentricity of the billet changes as it passes through all the production stages after cross-roll piercing or punch pressing. In this connection, the piercing operation, for example cross-roll piercing or punch pressing, is a forming operation in tube production that significantly influences eccentricity.

All the currently known technical methods are basically limited in their suitability for reproducibly reducing the eccentricity in the hole initiation zone of the billet to a very low value (E_(reiative)<2 to 3%). By means of the inventive method, eccentricity can be reduced markedly compared to the heretofore known technical methods. The order of magnitude of the improvement in eccentricity that can be achieved is, for example, a factor of 2 or greater, depending on the magnitude and pattern of the eccentricity in the reference hollow billet.

The material stability (yield stress) of steel materials is strongly dependent on temperature. Consequently, according to the invention the stability distribution in the billet material is influenced by targeted adjustment of a temperature distribution. The dependence of yield stress on the rate of shape change does not limit the effectiveness of the method according to the invention, since the rate of shape change generally remains approximately unchanged. Since a change in the temperature of the material of the billet is also accompanied by a change in its stability, in the inventive method a graded, rotationally symmetrical stability distribution can be created by external means, through local cooling and/or local heating, by varying in a targeted manner the temperature in at least the end face of the hole initiation end and the resulting temperature distribution in the end face of the billet. Preferably, a much lower temperature is created in the outer annular temperature change zone of the end face (that is, the outer region of the end face, also referred to as the outer temperature zone) than in the central zone, also referred to as the inner temperature zone, in the immediate vicinity of the central axis of the billet, which can also be referred to as the main axis or center axis of the billet.

According to the invention, the tool used for the piercing operation, particularly the inside tool of the forming unit, can be a piercing mandrel. In the inventive method, the piercing mandrel is, for example, able to penetrate more easily into the temperature zone formed in the center of the end face of the hole initiation end due to an increased temperature in that location. Since the temperature in the surrounding temperature zone is lower in this case and the stability in the surrounding zone is therefore higher, the piercing mandrel is centered automatically. Hence, in addition or as an alternative to heating in the center of the end side of the hole initiation end, the outer temperature zone surrounding a higher-temperature inner temperature zone can also be cooled, that is, the temperature in that zone can be lowered. This further reinforces the centering action.

The stability distribution, which is significantly influenced by the temperature distribution, thus improves centering action, especially during the first phase of the piercing operation (i.e., during the so-called “hole initiation”), in that the billet orients itself relative to the piercing mandrel by “self-centering.”

The term “hole initiation” is to be understood as the first phase of the piercing operation, that is, the time interval from the first contact of the piercing mandrel with the billet (or shortly before) to the stationary phase of the piercing operation.

The term “hole initiation zone” is to be understood as that zone in the billet which is pierced first during the piercing operation, that is, the front zone of the billet. This zone is not exactly geometrically defined in the strict sense, but is only a relatively vaguely delineated zone. With ordinary billet diameters of about D=200 mm, the hole initiation zone encompasses, for example, approximately the first interval length of 500 mm along the central axis of the billet, beginning at the end face of the billet. The term “hole initiation end of the billet” signifies that end of the billet which comprises the end face by which the piercing mandrel first comes into contact with the billet.

The term “self-centering action” is to be understood here as the effect that during the piercing operation—without additional external influences—the billet, as it is plastically deformed by the inside tool (here, the piercing mandrel), orients itself approximately centrically (that is, with only very slight lateral offset) relative to the inside tool through the basic mechanical principle of action and reaction, this taking place during the initiation of the hole in the billet.

An essential difference between the method according to the invention and the heretofore known, conventional measures or approaches for reducing eccentricity is that the centering is achieved by means of the temperature distribution, and thus by means of the stability properties in the billet to be pierced.

The essential advantage of the method according to the invention is that precise and reproducible centering of the piercing mandrel can be achieved during the piercing operation. In this way, the eccentricity of the hollow billet or of the tubular product can be reduced substantially compared to conventional methods or measures. The fact that according to the invention the temperature change zone in which a temperature change is effected is rotationally symmetrical to the central axis of the billet significantly assists the centering action.

The present invention has distinct advantages by virtue of the fact that the eccentricity in the hollow billet can be reduced significantly by means of the invention, compared to conventional methods, and can thus can be improved.

Using the method according to the invention, a hollow billet can be produced with a very uniform wall thickness distribution and consequently with very low eccentricity. In this way, the production of a hollow billet can in particular take place in a more reproducible and very precise manner.

Since a hollow billet of this kind can be used, for example, as a pre-stage for seamless tubing, the inventive method is suitable for the production of tubular pre-stages in the production of seamless tubing having a very uniform wall thickness distribution and thus very low eccentricity. By means of the method according to the invention, a hollow billet can be produced that presents minimal variations of eccentricity along the hollow billet, and qualitative and quantitative variations in the eccentricity patterns between different consecutively produced hollow billets can be substantially reduced.

The prevention of critical, excessive eccentricity values makes it possible to substantially reduce or even completely eliminate high reject counts and thus the major effort entailed in reworking the hollow billets or tubular products, with the attendant higher costs.

Hollow billets produced by the method according to the invention can also be used in particular for the production of precision tubes, particularly since the technical requirements imposed on the geometric characteristics of precision tubes (for example, on the uniformity of the wall thickness distribution and of the eccentricity distribution) are relatively high.

According to one embodiment, the temperature change is effected in a temperature change zone that is spaced apart radially from the central axis of the billet. This temperature change zone spaced apart radially from the central axis of the billet can, for example, extend to the outer contour of the billet. This embodiment is especially advantageous, since in addition to the targeted temperature change at the end face itself, a temperature change can also be initiated in a targeted manner over the length of the billet. For example, such a temperature change can be effected by acting on a portion of the length of the circumferential surface of the billet, which portion is adjacent the end side of the billet.

According to one embodiment, the temperature change that is effected in at least one temperature change zone at least at the hole initiation end of the billet is a decrease in temperature. As mentioned above, lowering the temperature causes an increase in stability compared to other temperature zones in the billet, and thus a centering and/or guiding of the piercing mandrel. Such a temperature decrease or targeted cooling of a temperature change zone of the billet can be accomplished, for example, by one or more of the following methods:

-   -   a) applying liquid media, for example water, preferably as a         coherent water jet,     -   b) applying gaseous media, for example steam, or     -   c) contact with a solid body, for example in the form of a         projection mask.

According to another embodiment, the temperature change zone in which the local temperature change is effected encompasses the central axis of the billet and the temperature change is an increase in temperature. This temperature change zone is also referred to as the inner temperature zone. Given a billet diameter of 210 mm, this temperature zone can have, for example, a diameter of 40 mm. The diameter of the inner temperature zone constituting the temperature change zone, i.e., in which the temperature is changed, can according to the invention be equal to 25%, preferably 20%, of the billet diameter. The diameter of the, for example, axially symmetrical tool (for example a piercing mandrel) that is introduced into this inner temperature zone and that comes into contact with the billet material during the first process phase of the piercing operation can be, for example, 30 mm. The diameter of the axially symmetrical tool (for example a piercing mandrel) in the zone of the tool that comes into contact with the billet material during a subsequent process phase of the piercing operation can be, for example, 130 mm. The temperature near the central axis of the billet and thus in the center of the inner temperature zone can be, for example, up to 1200° C., and the temperature in the outer temperature zone in the vicinity of the circumferential surface of the billet can be, for example, 950° C. These dimensions and temperatures are merely examples, and thus are not limitative of the inventive idea. The temperature difference adjusted between the outer face of the billet and the central axis, that is, the difference between an inner temperature zone and an outer temperature zone, can be, for example, 40%, preferably 20% of the temperature of the temperature zone having the highest temperature.

In the embodiment where an inner temperature zone having an increased temperature is created, the stability of the billet material is therefore reduced at the entrance site of a tool and particularly of a piercing mandrel, which normally is in the center on the end face and therefore encompasses the central axis of the billet, and the introduction of the tool, particularly of the piercing mandrel, into the billet is facilitated. The temperature increase can also be selected as sufficiently large so that as a result of this increased temperature, the billet material is melted in the temperature change zone and can therefore be removed from the end side of the billet prior to contact with the tool, particularly the piercing mandrel.

The temperature change effected according to the invention preferably takes place by the exertion of an external influence on the billet. The exertion of an external influence is referred to as action. The action is performed according to the invention on only a portion of the surface of the billet. To perform this action, for example, a jet can be directed at a portion of the surface of the billet. The jet can be a media jet or a beam of electromagnetic waves.

According to one embodiment, the local temperature change is effected by acting on a portion of one of the end sides of the billet. In this embodiment, the temperature change zone is formed solely by the end face of the billet. During this action on a portion of the end face of the billet, at least one other portion of the end face is not directly influenced by said action. If, for example, a jet is directed at the edge region of the end face, then the temperature change zone is formed in the edge region of the end face. In the center of the end face, however—i.e., in the region of the central axis of the billet—the previously adjusted temperature will remain virtually unchanged. Thus, by action on a portion of the end face of the billet, a temperature distribution can be exactly adjusted over the end face of the billet, particularly the hole initiation face and the hole initiation zone. Such exact adjustment of the temperature distribution in the end face and the hole initiation zone cannot be achieved reproducibly by acting solely on the circumferential surface of the billet. The end face of the billet on which a local temperature change is effected by such action can also be the hole exit face, in addition to the hole initiation face.

According to another embodiment, the local temperature change is effected by acting on a portion of one of the end faces of the billet and on a segment, adjacent to this end face, of the length of the circumferential surface of the billet, the length of the segment of the circumferential surface being smaller than the length of the billet. In this embodiment, the edge region of the end face of the billet preferably constitutes the temperature change zone. The length of the temperature change zone can be increased, in this embodiment, by additionally acting on the circumferential surface of the billet, as opposed to acting solely on a portion of the end face.

According to a preferred embodiment, the temperature change zone in which a temperature change is effected extends for no more than ⅓, particularly no more than ¼, of the length of the billet. In this way, for one thing, the desired temperature distribution that leads to centering of the piercing mandrel can be adjusted, for example in the hole initiation zone, while the rest of the billet continues to have a uniform temperature distribution, thus ensuring uniform reshaping after the introduction of the piercing mandrel.

According to one embodiment, a media jet is preferably directed at a portion of the hole initiation end, particularly of the hole initiation face, and, if appropriate, additionally to a portion of the hole exit end, particularly of the hole exit face, of the billet. Targeted adjustment of a temperature distribution at the particular end face of the billet can be ensured in this way. Alternatively or additionally, in order to subject a portion of the end side (or of both end sides) of the billet to a media jet, a portion of the length of the circumferential surface of the billet can also be impinged on by a media jet. In particular, this impingement can be used in the hole initiation zone of the billet and/or in the hole exit zone to effect a decrease in temperature at the circumferential surface for the purpose of guiding the tool, particularly the piercing mandrel. Alternatively to impingement with a media jet, a temperature change can also be effected according to the invention by changing the environmental conditions. Thus, for example, a projection mask or a cooling ring can be placed in front of a portion of the end face or arranged around a portion of the length of the circumferential surface, thus bringing about a decrease in the temperature of the billet material by convection.

According to a preferred embodiment, the metallic billet is a round billet. The advantages of the inventive method can be used especially advantageously with this billet shape, since billets of this kind are usually made into hollow billets that are intended to have a uniform wall thickness over the circumference so that they can subsequently be processed further into tubes. The temperature change zones in which a temperature change is to be made according to the invention thus do not vary in the circumferential direction of the billet. The adjustment of a desired temperature and thus of a desired temperature distribution is easier to ensure with temperature change zones having a shape of this kind.

According to a preferred embodiment, the billet is rotated about its own central axis at least before the effectuation and/or during the effectuation of the local temperature change.

The rotation of the billet about its central axis, which can also be referred to as the main axis or center axis of the billet, before the piercing operation particularly has the effect of centering the billet with respect to its central axis for the subsequent temperature change and particularly the adjustment of a temperature distribution on the end face. In this way, projection elements, which can also be referred to as temperature adjusting devices, can be reliably oriented with respect to the central axis of the billet and the creation of rotationally symmetrical temperature zones can be ensured.

Rotating the billet about its central axis during the effectuation of the local temperature change can be used in particular to remove billet material that has been melted by a temperature increase or coolants that have been applied to the end face from the end face of the billet by means of the centrifugal force effect produced by the rotation.

According to a preferred embodiment, the temperature change is effected by projecting at least one jet onto one of the end sides of the billet, and a projection element that is spaced apart from the billet is preferably used for projection.

“Projection” is to be understood here in particular as the transfer of a temperature distribution to the end face of the billet by targeted cooling or heating from the outside (from a reference object) in, for example, a direction orthogonal to the end face.

The projection should preferably take place in the region of the front end face, that is, the hole initiation face, and, if appropriate, also in the region of the rear face, that is, the hole exit face. “Projection element” refers to a device component by which at least one jet can be aimed at and conveyed to one of the end faces of the billet. The jet can include a media jet, that is, a medium such as air or water, for example. Alternatively, however, it is also possible for the jet to be a beam of electromagnetic waves.

The fact that the projection element is spaced apart from the billet enables the billet to be turned or continuously rotated, for example about its central axis, even as the temperature change is being effected. In addition, the use of a spaced-part projection element is also advantageous because, after projection, it can be moved relatively easily from the position on the center axis of the billet to another position, thus freeing up space, for example, for positioning the piercing mandrel or another tool at this location in front of the end face of the billet.

Through the use of the so-called projection element, the temperature distribution and thus the stability distribution, particularly on the end face of the billet, can be influenced in a targeted manner via the jet directed onto the billet by said projection element.

The projection element preferably includes at least one nozzle element. The nozzle elements are disposed on the projection element correspondingly to the temperature change zone whose temperature is to be varied by the jet. Preferably, the nozzle elements are annularly arranged or the projection element comprises a single annular nozzle. The resulting annular projection element is preferably oriented centrically with respect to the central axis of the billet. The nozzles of the nozzle element are oriented such that they emit a jet parallel to the central axis of the billet and thus also parallel to the central axis of the projection element. In this way, particularly an annular temperature change zone spaced apart from the central axis can be impinged on by the jet or jets, thus resulting in a defined decrease in temperature in this temperature change zone. The temperature change zone in this case is rotationally symmetrical to the central axis of the billet.

According to one embodiment, at least one jet of coolant, and particularly preferably a water jet, is directed at the end side, particularly at a portion of the end face, of the billet. The water jet can be directed at the end face of the billet by a projection element constituted by an annular element comprising water nozzles. In addition, it is also possible to direct a water jet onto the circumferential surface of the billet in the region of the end face. This can take place, for example, by means of a nozzle element that is arranged annularly around the billet and on which nozzles are aimed radially inward.

According to another embodiment, at least one jet, particularly an oxygen jet, is directed onto the end side of the billet to generate an exothermic reaction at the end face and in the adjacent interior of the billet. In this embodiment, the jet is preferably directed onto the region around the central axis of the billet. The jet can in this case be directed at a region that is located solely in the central axis of the billet or that additionally encompasses another region around the central axis. By means of the exothermic reaction, the temperature in this temperature change zone is increased to such an extent that the billet material is melted in this temperature change zone. To remove the melted billet material from the billet, the latter is preferably rotated about its central axis at high rotation speeds, thus creating a void (cavern) that is open to the end side and is approximately rotationally symmetrical to the central axis. This void serves as a pre-hole for a piercing mandrel to be introduced subsequently or for another tool, and thus centers the piercing mandrel or tool. One advantage of this method of sinking a pre-hole is that no other materials that might react with the billet material are added to the billet. This is a risk if a carbon arc is used to cause melting in the end face, in which case carburization of the billet material and thus increased brittleness are likely.

According to an alternative embodiment, at least one electromagnetic beam that is reflected from the end side of the billet via at least one reflector is directed at the end side of the billet. In this way, a warming, that is, a heating, of the central zone of the end face (and thus a temperature increase) in the immediate vicinity of the central axis of the billet can be effected by means of a radiation reflector disposed at a defined axial distance from the particular end face of the billet. Preferably only one radiation reflector is used, to concentrate a beam onto the end side of the billet and particularly onto the central zone of the end face. The distance of the reflector from the end face is to be designed according to the wavelength of the electromagnetic radiation. The electromagnetic beam directed onto the end face is preferably a reflection of the thermal radiation from the end face. This embodiment has the advantage of eliminating the need to additionally provide a medium, such as oxygen, for obtaining a temperature increase at the end face. If the heat radiation from the billet is used to increase the temperature in at least one temperature change zone of the billet, the temperature in the heated temperature change zone will usually be below the melting temperature of the billet material and thus will not melt it. However, such a temperature increase, particularly in the central zone of the end face, still ensures centering of the tool, particularly the piercing mandrel, and guidance of the tool, particularly the piercing mandrel, since the temperature of the surrounding temperature zone is lower than the temperature in the central zone constituting the temperature change zone.

According to one embodiment, a temperature change is also effected at the hole exit end of the billet, i.e., the opposite end from the hole initiation end, and takes place at least in the end face thereof. The end face of the hole initiation end is also referred to as the front end face. The end face of the hole exit end is also referred to as the rear end face. The temperature distribution that is adjusted at the hole exit end can be the same as or different from the temperature distribution at the hole initiation end. In addition, the temperature distribution on the hole exit end of the billet can be created in the same way as or in a different way from the temperature distribution on the hole initiation end of the billet. For example, a temperature increase can be generated at the hole initiation end by means of an oxygen jet and at the hole exit end by means of a reflected beam of thermal radiation.

By changing the temperature at the hole exit end, guidance of the piercing mandrel or another tool can be accomplished even during the final phase of the piercing operation. Low eccentricity is thereby ensured even in the hole exit zone, that is, near the end side of the hole exit end.

According to another aspect, the invention is directed to a device for producing a hollow metallic billet from a metallic billet, which device comprises a holder for the billet. The device is characterized in that it includes at least one projection element operative to change the temperature of the billet in the holder at least zonewise and directed at a portion of at least one of the end sides of the billet.

The holder for the billet can be one-part or multi-part. In particular, the holder can comprise rolls that are able to engage the circumferential surface of the billet and rotate it about its central axis. In addition, the holder can comprise at least one forming unit serving to make a hole in the billet.

For this purpose, the device can include, for example, a cross-roll piercing mill or a punch press. In addition, the device for producing a hollow billet comprises at least one tool, preferably in the form of a piercing mandrel. The tool, particularly the piercing mandrel, can be configured as a separate component and can, for example, be one device with the forming unit. According to the invention, the holder for the billet can be provided in the form of additional equipment, for example a cross-roll piercing mill or a punch press, and can either be integrated therein or provided separately therefrom. In the first case, it is also possible for the holder to be formed by a portion of, for example, the cross-roll piercing mill or the punch press.

According to the invention, the device comprises at least one projection element, which can also be referred to as a temperature adjusting device, which is operative to change the temperature of the billet in the holder at least zonewise and is directed at a portion, i.e. a subregion, of at least one of the end sides of the billet.

The device according to the invention differs in this regard from a furnace, which is not capable of heating a portion or a subregion of an end side in a targeted manner.

The projection element is preferably spaced apart from the billet holder and from the billet. The projection element is preferably designed so that it can be moved relative to the billet holder. In this way, for one thing, a desired or defined spacing can be set during the adjustment of the temperature at the billet. For another thing, in this embodiment the projection element can also be removed from the holder, for example in order to guide the piercing mandrel to the billet. Alternatively, however, it is also possible for the billet to be removed from the holder for the piercing operation and placed in a forming unit, for example a cross-roll piercing mill or a punch press, without the need to move the projection element.

Particularly preferably, the projection element is aligned with the central axis of the billet in the holder. This makes it possible to treat target zones that are rotationally symmetrical to the central axis and in particular to change the temperature in that zone or those zones.

The projection element can, according to the invention, be for example a nozzle or a nozzle ring. Alternatively, a reflector, for example, can also be used as a projection element. It is also, however, within the scope of the invention to use a different kind of temperature adjusting device as a projection element, as long as this temperature adjusting device is capable of effecting a regional or zonewise change in temperature at least at the surface of the billet.

The projection element can include at least one nozzle or at least one reflector. According to the embodiment in which the projection element includes nozzles, either a single nozzle is used, which is preferably directed at the center of the end face, or a plurality of nozzles is used, which are arranged parallel to and displaced outwardly from the central axis. In the first case, a medium, for example oxygen, for generating an exothermic reaction in the end face of the billet can be delivered to the billet. In the second case, for example water or another cooling fluid can be used to lower the temperature at the zone of the end face.

If a reflector is used, then it preferably serves to return the thermal radiation emitted by the billet from the end side to the end side. The reflector preferably has a curved shape, as a result of which the beams striking the reflector can be deflected back, concentrated by the curvature, to certain zones of the end side of the billet.

The projection element can be disposed at the hole initiation end and/or at the hole exit end of the billet. A projection element is said to be disposed at one of the ends in this context if it is directed at the particular end, particularly at the particular end face, but is spaced apart therefrom, i.e., is able to act on the particular end.

The device according to the invention is preferably designed to carry out the method according to the invention.

Advantages and features described with reference to the method according to the invention apply correspondingly—insofar as they are applicable—to the device according to the invention and vice versa. Advantages and features will therefore be mentioned only once if appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described again below with reference to the preferred embodiments and the appended drawings. Therein:

FIG. 1 is a schematic view of an embodiment of the device according to the invention for carrying out an embodiment of the method according to the invention;

FIG. 2 is a schematic view of another embodiment of the device according to the invention for carrying out another embodiment of the method according to the invention;

FIG. 3 is a schematic view of another embodiment of the device according to the invention for carrying out another embodiment of the method according to the invention;

FIG. 4 is a schematic illustration of a billet in a holder according to one embodiment of the device according to the invention;

FIG. 5 is a schematic illustration of the eccentricity pattern along a pierced billet according to the state of the art;

FIG. 6 are schematic illustrations of a billet with and without an inserted piercing mandrel;

FIG. 7 are schematic illustrations of a billet with a piercing mandrel;

FIG. 8 is a schematic illustration of the temperature distribution in an end face of the billet; and

FIG. 9 is a schematic illustration of the temperature distribution over the length of the billet.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment of the device according to the invention for carrying out an embodiment of the method according to the invention. In this case, the billet 1, which in the embodiment shown is a round billet, is held on a holder (not shown), and can by means of this holder be set in rotation about the central axis M. The direction of rotation is indicated in FIG. 1 by the arrow labeled ROT.

An embodiment of the holder that can be used to hold the billet 1 is illustrated schematically in two views in FIG. 4. As can be seen from this FIG. 4, in the embodiment shown the holder 3 consists of a set of three rolls 30. In the illustrated embodiment, one roll 30 is a drive roll 300 and two other rolls 30 are guide rolls 301. The billet 1 thus is driven by the drive roll 300, whose axis of rotation or central axis MR is parallel to the central axis M of the billet 1, and is guided in its rotational movement by the guide rolls 301, whose center axes MR are also parallel to the central axis M of the billet 1.

A projection element 2, which can also be referred to as a temperature adjusting device, is also provided in the embodiment according to FIG. 1. In the embodiment, this projection element 2 is an element comprising annularly arranged nozzles 20. However, it is also possible for the projection element 2 to include a single annular nozzle 20. The nozzles 20 of the projection element 2 are directed at one of the end faces of the round billet 1. This end face is preferably the end face of the hole initiation end 11 of the billet 1, that is, the end from which the hole is made. The region of the length of the billet 1 that is adjacent to this end face is also referred to as the hole initiation zone 13. The opposite end face is therefore located on the hole exit end 12 of the billet 1. The hole exit end 12 refers to the end of the billet from which the piercing mandrel emerges.

In the illustrated embodiment, the projection element 2 serves to apply a coolant, for example water, to the heated billet 1. The water jet 21 here is directed to a temperature change zone on the end face of the hole initiation end 11, displaced radially outward from the central axis M of the billet 1. The temperature zones 110 and 112 indicated in FIG. 1 are formed in this way. The inner temperature zone 110 in this case has a higher temperature than the outer temperature zone 112 surrounding it, which constitutes the temperature change zone.

The coolant, particularly water, that is applied by the projection element 2 to the end face of the hole initiation end 11 is carried off, by the rotation (ROT) of the billet 1 and the resulting centrifugal force, from the end side of the hole initiation end 11 in the paths indicated by curved arrows in the figure.

A tool, depicted in FIG. 6 as a piercing mandrel 4, can now be introduced into the thus prepared billet 1 at the hole initiation end 11. As a result of the adjusted temperature distribution, this piercing mandrel 4 is centered automatically and guided along the central axis M. In this embodiment, a temperature distribution is created in which the temperature is higher in the inner temperature zone 110 than in the outer temperature zone 112. The temperature in the inner temperature zone 110 in this case essentially corresponds to the billet temperature to which the entire billet is heated before the application of the method according to the invention. By contrast, the temperature in the outer temperature zone 112, i.e. the temperature change zone, is reduced in comparison to this billet temperature.

The embodiment shown in FIG. 2 differs from the first embodiment, shown in FIG. 1, merely by the type of projection element 2. In the embodiment illustrated in FIG. 2, the projection element 2 applies oxygen to the end face of the hole initiation end 11 of the billet. In the embodiment shown, the projection element 2 is configured with an annular nozzle 20. However, it is also possible in this embodiment to use a nozzle that is directed at the center, i.e., the central axis M, of the billet 1. The oxygen serves to effect a temperature increase around the central axis M at the end side of the hole initiation end 11, in the central region of the end face. This temperature increase is large enough so that the billet material is melted in the region of the central axis M at the hole initiation end 11. By the rotation (ROT) of the billet 1 and the resulting centrifugal force, the melted billet material is carried off from the end side of the hole initiation end 11 in the paths shown as curved arrows in FIG. 2. This produces a recess 111, also referred to as a cavern, in the hole initiation end 11 of the billet 1. A holder 3 of the billet 1, corresponding to the embodiment illustrated in FIG. 4, can also be provided according to the embodiment of FIG. 2. In addition to the formation of a recess 111, in this embodiment a temperature distribution is created according to which the temperature is higher in the inner temperature zone 110, which in this case constitutes the temperature change zone, than in the outer temperature zone 112. The temperature in the outer temperature zone 112 here corresponds essentially to the billet temperature to which the entire billet is heated before the application of the method according to the invention. The temperature in the inner temperature zone 110, however—i.e., the temperature change zone—is increased in comparison to this billet temperature.

FIG. 3 shows another embodiment of the device for carrying out an embodiment of the method according to the invention. In this embodiment, one reflector 22 each is disposed in front of the hole initiation end 11 and the hole exit end 12. The reflectors 22 are located at a distance from the end sides of the billet 1 at the hole initiation end 11 and the hole exit end 12. The holder 3 of the billet 1 is implemented similarly to holder 3 in FIG. 4. The reflectors 22 each have a concavely curved shape. Owing to this shape of the reflectors 22, the thermal radiation emitted from the hole initiation end 11 and from the hole exit end 12 is reflected back to the respective end 11, 12. The reflected radiation is, in this case, concentrated particularly and preferably in the center of the end face of the hole initiation end 11 and of the hole exit end 12. Thus, in this embodiment as well, a temperature distribution is obtained according to which the temperature is higher in the inner temperature zone 110 than in the outer temperature zone 112. The temperature in the outer temperature zone 112 here corresponds essentially to the billet temperature to which the entire billet is heated before the application of the method according to the invention. The temperature in the inner temperature zone 110 constituting the temperature change zone, however, is increased in comparison to this billet temperature.

Combinations of the illustrated embodiments of the method are also possible. For example, a temperature increase in the inner temperature zone 110 and a temperature decrease in the outer temperature zone 112 can be effected simultaneously. In this case, both temperature zones 110, 112 constitute temperature change zones, although the temperature changes in the two zones are different. Specifically, in this case the temperature change in the one temperature change zone 110 is an increase in temperature and in the other temperature change zone 112 it is a decrease in temperature.

The principle used by the inventive method and on which the invention is based is to project a graded temperature distribution that is rotationally symmetrical to the central axis of the billet at least onto the end face of the billet and thus to create a corresponding yield stress distribution in the billet, particularly in the hole initiation zone of the billet. This temperature and stability distribution in the billet serves as a quasi-internally-created, centric and rotationally symmetrical guide ring that guides the piercing mandrel centrically with respect to the central axis of the billet during the piercing operation. This guidance is effected by means of a virtually invisible force action across the defined stability distribution created in the billet. The temperature distribution and force actions in the billet are shown schematically in FIGS. 6 to 9. FIG. 7 schematically indicates the force vectors acting on the piercing mandrel to center it in the billet. The arrows in FIG. 7 indicate the reaction force acting on the mandrel. FIG. 8 schematically shows two exemplary zone radii R′, R″ of the inner temperature zone 110, which define two boundaries of characteristic exemplary temperature zones on the end face.

In this connection, the radial profiles of the temperature gradients to be adjusted can be configured in different ways. It is inherently the case, as a basic principle, that the steeper the chosen rotationally symmetrical gradation of the temperature distribution—i.e., the greater the relevant temperature gradient in the radial direction with respect to the central axis of the billet—the stronger the guiding action during the piercing operation tends to be.

The central axis of the billet, also called the main axis of the billet or the center axis of the billet, is more or less the axis of symmetry in the central zone of the billet and is to be understood as the central axis without, for example, the squeezed ends of the billet that can form during its production. The geometry of a cylindrical billet is, by nature, never ideally rotationally symmetrical in reality. This assumption of an ideal rotational symmetry for the geometry of the billet, which in reality is only approximately valid, is adequate for the effectiveness of the method according to the invention. The fact that the rotational symmetry of the billet is not strictly perfect in reality does not limit the effectiveness of the inventive method.

The temperature distribution that is adjustable according to the invention makes it possible to achieve the following effects during the subsequent or simultaneous piercing of the billet:

1) Guidance at the beginning of the piercing operation by means of a graded, defined, rotationally symmetrical temperature distribution on the hole initiation end, i.e., the front end face, of the billet.

2) Centric guidance of the piercing mandrel at the end of the piercing operation by the creation of a graded, defined, rotationally symmetrical temperature distribution on the hole exit end, i.e. the rear end face, of the billet.

3) Centric guidance of the piercing mandrel throughout the piercing operation by the creation of a graded, defined, rotationally symmetrical temperature distribution on the entire circumferential surface of the billet.

To create the desired centering force action on the piercing mandrel, that is, in the direction of the central axis of the billet, according to the method of the invention the temperature distribution on the end face of the billet, and thus the distribution of the yield stress of the billet material, are adjusted so that the highest temperature and thus the lowest yield stress within the billet are on the central axis. With increasing radial distance from the central axis, according to the inventive method the temperature level of the temperature distribution to be adjusted should decrease and the yield stress therefore basically increase. The temperature zone that forms around the central axis, and in which the temperature is higher than in the outer edge zone and the billet material therefore has a lower yield stress, causes the billet material to deform plastically around the piercing mandrel so as to produce a material distribution having the greatest possible rotational symmetry, and thus the lowest possible eccentricity of the pierced billet and the most uniform possible wall thickness distribution.

The underlying mechanism here is based on the fact that the rotational symmetry of the graded, defined temperature distribution, and thus a corresponding stability distribution in the billet to be pierced, have the effect of causing a centering action. The mechanism or mode of action is based on two decisive characteristics: the rotational symmetry of the stability distribution in the billet and the radial gradient to the stability distribution in the billet.

The rotational symmetry of the stability distribution in the billet causes the restoring force to act in the proper direction, i.e., the radial direction, toward the central axis of the billet. The radial gradient of the stability distribution in the billet causes the restoring force acting primarily in the radial direction to be of sufficient magnitude, and the positional correction thus to take place with sufficient speed.

In the case of a large radial gradient, the effect achieved is that even in response to a small deflection of the piercing mandrel, the resultant restoring force in the opposite direction from the deflection, i.e. extending radially to the central axis of the billet, is relatively great and thus has a strong effect. Effective and rapid correction of the position of the piercing mandrel is obtained in this way.

The method according to the invention can be applied both to cross-roll piercing (usually with a non-driven rotating piercing mandrel, a mandrel bar and driven rotating rolls) and to punch pressing (usually with solely translational movement by the driven piercing mandrel). The diameter of the temperature zone where a high temperature prevails is preferably selected as a function of the diameter of the billet and as a function of the diameter of the billet and the piercing mandrel or mandrel tip, for example as follows:

D _(centering zone)=app. 0.3 to 1.2×D _(mandrel)

D _(centering zone)=app. 1.0 to 3.0×D _(mandrel tip)

D _(centering zone)=app. 0.1 to 0.7×D _(billet)

The following considerations must be taken into account with regard to the geometric characteristics of the billet. The billets are usually cut from the bloom, for example by the parting methods of hot shearing or sawing. Depending on the parting method used, the end face of the billet thus is not usually ideally circular, but deviates slightly from the ideal circular shape to a greater or lesser degree.

In addition to the shape of the end face, the position of the end face is also important, especially if the end face has been squeezed. For example, the center of the end face of the billet—both of an ideally circular and of a not ideally circular end face—can be outside the central axis of the billet, that is, it can be displaced laterally in the radial direction. Since, as a rule, the geometry of the end face is not exactly circular, according to the inventive method the definitive reference to be used in centering the temperature distribution to be created is preferably the central axis of the billet, rather than the end face of the billet.

A number of methods can theoretically be used to determine the central axis for centering the billet; for example, this can be done visually, mechanically, with the aid of other physical effects or combinations of effects. In the case of the inventive method, the centering is preferably to be accomplished in the following manner:

By rotating the billet, for example by means of a drive roll and a bearing for free rotation (see FIG. 4).

To achieve centering in this case, it is essential that the projection element preferably used according to the invention, for example a nozzle or a reflector, be positioned sufficiently precisely relative to the billet. One practical approach is to select the previously determined position of the central axis of the billet as the target position for aligning the projection element according to the invention. The exact position of the central axis of the billet can be determined in this case as the center of the axes of rotation of the drive and guide rolls (see FIG. 4). This thereby-known position of the central axis of the billet then serves as an input for the controlling or regulating arrangement used to position the projection element according to the invention. Such control or regulation can be effected, for example, via electronically assisted mechanical coupling of the centering elements (that is, the drive and guide rolls) to the projection element or reflector according to the invention.

The rotational speed of the billet during the centering process according to the method of the invention can be, for example, a rotational speed in the range n=200 to 800 min⁻¹=app. 3.3 to 13.3 s^(−1. At rotational speeds of this kind, the coolant, for example cooling water, or the melted particles of billet material can be carried off reliably and with process stability.)

The duration of the projection operation (cooling and/or heating; for example, spraying with water) can be, for example, in the range of t=5 to 15 s.

The pressure (water pressure) used for the projection operation (cooling, for example spraying with water), i.e. the water pressure, is, for example, a pressure in the range of p=app. 6 to 200 bar.

The size of the nozzle opening used (cross-sectional area, where applicable a circular cross section by diameter) is, for example, a diameter of D=1 to 5 mm.

The velocity of the water during the projection operation (cooling) can be calculated from the above values.

The temperature field varies spontaneously—even without external influences—as a result of thermal conduction and thermal radiation, and the temperature level in the billet as a whole therefore decreases with advancing process time. To ensure that the created temperature field is preserved adequately until the piercing of the billet is initiated, care must be taken that the period of time between the end of the projection operation and the start of the piercing operation does not exceed a critical limit.

The time chosen to elapse between the end of the projection operation and the start of the piercing operation can be, for example, t<app. 20 s.

The length, in the direction of the central axis of the billet, of the temperature change zone in the region of the end face onto which the temperature field is to be projected can be, for example, z_(temperature field)=0 to 100 mm.

To prevent attenuation of the temperature distribution, if the billet is to be descaled after it is removed from the furnace, the creation of the temperature distribution (“projection”) must take place after the descaling.

The underlying physical processes are highly complex. However, the suitable parameters of the process (for example durations, instants, temperature curve) can be approximated on the basis of theoretical considerations and by numerical simulation (finite elements method).

The necessary physical variables, for example some material constants of the steel billet, can be assumed in making this rough calculation.

Physical effects which it is essential to consider in the calculation are:

-   -   thermal conduction     -   thermal radiation

Thermal diffusivity (symbol “a”) is defined as follows:

$a = \frac{\lambda}{\rho \cdot c_{p}}$

by with the physical variables thermal diffusivity λ, density ρ, specific heat capacity c_(p). The unit for thermal diffusivity a is m²/s. The thermal diffusivity can be used to make an approximate determination of the velocity with which a temperature front moves within the object. The thermal diffusivity of steel is approximately a (steel)=12 to 15×10⁻⁶ m²/s.

Based on the mathematical differential equations for the propagation of temperature in a solid body by thermal conduction alone (differentiation between a cylindrical rod of infinitesimally small diameter and infinite length and a cylinder of infinite diameter D and finite length L) and for the variation of temperature due to thermal radiation, an estimate can be made of temperature variation over time.

The parameters of the projection operation according to the inventive method must be suitably adjusted to and optimized for the particular use case by means of the particular boundary conditions of the process chain in production.

Alternatively or additionally to the described cooling of a temperature change zone of the billet, which is illustrated schematically in FIG. 1, so-called thermal centering can also be utilized according to the invention by heating the billet and creating a centering guiding action in the billet. Different variants (for example as illustrated in FIG. 2 and FIG. 3) of the method can be used for this purpose. For example, heating can be performed by initiating an exothermic reaction between oxygen and the metallic billet material. This can be carried out, for example, by means of the device illustrated in FIG. 2. Alternatively, the thermal radiation from the billet can be employed by using a reflector to heat a portion of the billet and particularly a temperature change zone of the end side of the billet. This can be performed, for example, by means of the device illustrated in FIG. 3.

The first method variant, in which heating is effected by initiating an exothermic reaction between oxygen and the metallic billet material in order to bring about a temperature change, makes use of the fact that iron burns in an oxygen atmosphere. The ignition temperature of steel is about 1200° C. The exact ignition temperature of steel depends on its carbon content and its content of other alloying or accompanying elements.

The exothermic reaction begins at the ignition temperature specific to the material, which in the case of iron is well below 1200° C. For this reason, it is possible to utilize the quantity of heat that is present in the billet, owing to its temperature level of over 1200° C., to carry out this reaction continuously for a given period of time. This evolved heat is transferred to the zone in the immediate vicinity of the central axis of the billet. This increases the temperature in that zone. The temperature increase in this case is large enough that quantities of metal and oxides are melted out of the billet material and according to the method of the invention are spun outward, particularly radially outward, by the centrifugal forces generated by the rotation of the billet. At the same time, liquid metal is flushed or spun out of the billet along with oxides. The quantity of heat in the rest of the billet decreases only slightly as a result, and remains sufficiently high for the ignition process that is still taking place to run its course in the rest of the billet material.

Thus, given a suitably large increase in the temperature of the particular end face zone of the billet, a cavern-shaped void forms, open to the end side and having an approximately elliptic paraboloid geometry; this void can also be referred to as a recess. This cavern causes centric guidance on the central axis of the billet during the hole initiation phase. This reduces the lateral deflection of the piercing mandrel during the piercing process and limits the lateral deflection to a very low value.

For this purpose, oxygen is jetted onto the end face of the billet through a nozzle disposed in front of said end face. If the temperature of the billet is above 1200° C., a chemical reaction (here, chemical ignition) of iron with oxygen ensues. As a result, the iron from the hole initiation end of the billet melts near the central axis of the billet. Due to the relatively rapid, forced rotation of the billet, the liquid iron is flushed or spun out, particularly as a result of the centrifugal forces, and leaves a cavern-shaped void, open to the end side of the billet and having an approximately elliptic paraboloid geometry, where the temperature, owing to the exothermic reaction, is higher than the previous temperature on the end face of the billet.

The time selected for this heating process can be a period of, for example, t=2 to 20 s.

As an alternative to the above-described exothermic reaction brought about by the addition of oxygen, a reflector (for example of copper or aluminum, coated and water-cooled) for reflecting electromagnetic radiation (FIG. 3) can be used as a projection element. A respective reflector of this kind can be disposed in front of each of the two end faces of the billet. The distance of the reflector from the end face is to be selected according to the wavelength of the electromagnetic radiation. In FIG. 3, z₁ and z₂ denote the axial distances of the reflectors from the end sides of the billet. To describe the position of the reflectors unambiguously in three-dimensional space, for example two angles (Θ₁, Φ₁) can be used for reflector 1 and two angles (Θ₂, Φ₂) for reflector 2. The respective angle pairs (Θ₁, Φ₁) and (Θ₂, Φ₂) here denote the two inclination angles of the axis of reflector 1 and of reflector 2, respectively, relative to the applicable two spatial axes of a Cartesian coordinate system.

The essential requirement for obtaining this effect, with the use of a reflector of this kind, is the electromagnetic radiation emitted by the end face of the billet due to the high temperature (T>1000° C.). The operating principle of the reflector is that the reflector reflects the electromagnetic radiation emitted by the end face of the billet back to that end face and thus influences the temperature distribution on that end face.

As a resulting effect, the end face of the billet heats up, specifically at the locations on the end face at which the reflector is aimed. The heat will spontaneously propagate continuously in the billet and in the ambient air according to the physical mechanisms of thermal conduction and thermal radiation, and the temperature distribution will, likewise continuously, vary slightly to a greater or lesser degree. It should be noted here that precise prediction of the temperature distribution in the billet over time is not an essential requirement for the mechanism of action of the inventive idea.

With respect to the quantity of reflected electromagnetic radiation, the reflector can be designed in such a way that in the presence of a suitably large quantity of reflected radiation, significant heating of the end face of the billet results, or in the presence of a suitably small quantity of reflected radiation, slower cooling of the end face of the billet results.

A graded, defined distribution of the quantity of radiation can be adjusted or achieved via the contour of the reflector and the distance of the reflector from the end face of the billet. The choice of a different or modified geometric contour for the reflector makes it possible to obtain a different optical reflection effect. A suitable reflector contour (geometry) should be designed and used, according to the reflection effect desired (i.e., the intensity of the heating brought about by the resulting temperature distribution, primarily in the end face zone of the billet).

This effect is obtained, in particular, even without the addition of oxygen, and thus without an exothermic reaction. In principle, therefore, there is no formation of a void (cavern) open at one end.

Due to the strong temperature dependence of the heat flow ∂Q/∂t (thermal radiation), which is described by the Stefan-Boltzmann law, with a billet temperature above 1000° C. the heat flow or electromagnetic radiation (especially the infrared radiation) is very high and increases sharply with increasing temperature. Consequently, at such high temperatures (T>1000° C.) the temperature increase in the billet relative to the ambient temperature is substantial and therefore substantially influences the temperature distribution in the billet. In this way, a temperature increase can be obtained (in a defined, desired manner in the sense of the inventive idea) in the center zone of the end face of the billet.

Heat Flow:

(Stefan-Boltzmann  law) $\overset{.}{Q} = {\frac{\partial Q}{\partial t} = {{ɛ\sigma}\; A\; T^{4}}}$

with the physical variables:

-   -   heat flow (radiant flux)

$\frac{\partial Q}{\partial t}\left( {{unit}\text{:}\mspace{14mu} {joules}\mspace{14mu} {per}\mspace{14mu} {second}} \right)$

-   -   emissivity & (values between ε=0 for a “perfect reflector” and         ε=1 for an “ideal black object”),     -   Stefan-Boltzmann constant

${\sigma = {{5.67 \cdot 10^{- 8}}\frac{W}{m^{2}K^{4}}}},$

-   -   area of the emitting body A and     -   temperature of the emitting body T (in kelvin units).

As delineated in FIG. 3, the reflectors should be adjustable (i.e., controllable) individually or together and, if appropriate, regulable during the process. This adjustability can be obtained with the necessary kinematic geometric degrees of freedom (for example, the distance of the reflector from the end face of the billet; the particular inclination angle of the reflector axis relative to the particular spatial axes, for example relative to two orthogonal coordinate axes; see FIG. 3) by defined positioning and mounting. This ability to adjust position in a defined manner via the location and orientation of the particular reflector, combined with the contour geometry selected for the reflector, makes it possible to achieve defined adjustment of the reflector and thus to obtain a defined effect from the reflection of radiation and consequently to obtain a defined heat and temperature distribution in the billet end face and the adjacent interior of the billet.

The lower the heating of the particular end face zone of the billet, the less pronounced will be any cavern-shaped void, open to the end side, that forms. If the heating is sufficiently low, the void that forms will be only negligibly small.

FIG. 5 is a simplified representation of a typical, theoretical pattern of relative eccentricity along a pierced billet according to the prior art. This pattern is the result of statistical analysis of a large number of rolling operations performed. The eccentricity of the pierced billet is plotted on the y-axis and the axial coordinate in the hollow billet is plotted on the x-axis. L here denotes the length of the hollow billet.

In the case of billets not made from low-alloy or unalloyed steels, melting (ignition, exothermic reaction) by oxygen jet is either impossible or cannot be achieved without additional measures. For this reason, part of the method according to the invention—specifically, the part comprising the heating of the temperature change zone around the central axis of the billet using an oxygen jet—is limited to the spectrum of application involving billets made from low-alloy or unalloyed steels.

The other part of the method according to the invention, however, comprising the cooling of the outer temperature change zone of the end face of the billet, includes billets made from any metals as a possible spectrum of application.

In the case of low-alloy and/or unalloyed steels, the centering action can be obtained by a combination of both projection methods, i.e., by cooling the billet zone (primarily on the end face of the billet) (FIG. 1) outside the central axis of the billet and heating (“thermal centering” by heating) the billet zone in the zone near the central axis, particularly on the end face of the billet (FIGS. 2, 3).

To prevent the temperature from being decreased too much and having a deleterious effect during the subsequent rolling operation, the local temperature projection (i.e., local cooling and/or local heating on the end face of the billet to be pierced) must be completed within a suitable process window.

List of Reference Numerals

1 Billet

10 Hollow billet

100 Pierced hole

11 Hole initiation end

110 Inner temperature zone

111 Recess

112 Outer temperature zone

12 Hole exit end

13 Hole initiation zone

2 Projection element

20 Nozzles

21 Jet of medium

22 Reflector

3 Holder

30 Rolls

300 Drive roll

301 Guide roll

4 Piercing mandrel

M Central axis (billet/hollow billet)

MR Central axis (roll)

ROT Direction of rotation (billet)

r Radius (variable)

R Radius (billet)

R′ Zone radius (temperature field)

R″ Zone radius (temperature field) 

1. A method for producing a hollow metallic billet (10) from a heated metallic billet (1) by a piercing operation, characterized in that a local temperature change is effected at least at the hole initiation end (11) of the billet (1) in at least one temperature change zone (110, 112) and the temperature change zone is rotationally symmetrical to the central axis (M) of the billet (1).
 2. The method as in claim 1, characterized in that the temperature change is effected in a temperature change zone (112) that is spaced apart radially from the central axis (M) of the billet (1).
 3. The method as in claim 1, characterized in that the temperature change is a decrease in temperature.
 4. The method as in claim 1, characterized in that the temperature change zone (110) in which the local temperature change is effected encompasses the central axis (M) of the billet (1) and the temperature change is an increase in temperature.
 5. The method as in claim 1, characterized in that the local temperature change occurs as a result of action on a portion of one of the end faces of the billet (1).
 6. The method as in claim 1, characterized in that the local temperature change occurs as a result of action on a portion of one of the end faces of the billet (1) and a segment, adjacent to said end face, of the length of the circumferential surface of the billet (1), the length of the segment of the circumferential surface being less than the length of the billet (1).
 7. The method as in claim 1, characterized in that the temperature change zone (110, 112) in which a temperature change is effected extends for no more than ⅓, particularly no more than ¼, the length of the billet (1).
 8. The method as in claim 1, characterized in that the billet (1) is a round billet.
 9. The method as in claim 1, characterized in that the billet (1) is rotated about the central axis (M) at least before the effectuation and/or during the effectuation of the local temperature change.
 10. The method as in claim 1, characterized in that the temperature change is effected by projecting at least one jet (21) onto one of the end sides of the billet (1), and a projection element (2) that is spaced apart from the billet (1) is preferably used for said projection.
 11. The method as in claim 1, characterized in that least at one water jet is directed at the end side of the billet (1).
 12. The method as in claim 1, characterized in that least at one jet (21), particularly an oxygen jet, is directed at the end side of the billet (1) to generate an exothermic reaction at the end face or in the billet (1).
 13. The method as in claim 1, characterized in that least at one electromagnetic beam (21) is directed at the end side of the billet (1) and is reflected from the end side by at least one reflector (22).
 14. The method as in claim 1, characterized in that a temperature change is also effected at the hole exit end (12) of the billet (1), i.e., the opposite end from the hole initiation end (11).
 15. A device for producing a hollow metallic billet (10) from a metallic billet (1) and comprising a holder (3) for the billet (1), characterized in that said device includes at least one projection element (2) operative to change the temperature of the billet (1) in the holder (3) at least zonewise and directed at a subzone of at least one of the end sides of the billet (1).
 16. The device as in claim 15, characterized in that the projection element (2) is aligned with the central axis (M) of the billet (1) in the holder (3).
 17. The device as in claim 15, characterized in that the projection element (2) includes at least one nozzle (20) or at least one reflector (22).
 18. The device as in claim 15, characterized in that the projection element (2) is disposed at the hole initiation end (11) and/or at the hole exit end (12) of the billet (1).
 19. A device for producing a hollow metallic billet (10) from a metallic billet (1) and comprising a holder (3) for the billet (1), characterized in that said device includes at least one proiection element (2) operative to change the temperature of the billet (1) in the holder (3) at least zonewise and directed at a subzone of at least one of the end sides of the billet (1), characterized in that it is designed to carry out the method as in claim
 1. 